Preferable radical polymerization initiators include azo compounds such as 2,2???-azobisisobutyronitrile, 2,2???-azobis(2,4-dimethylvaleronitrile), dimethyl-2,2???-azobisbutyrate, 2,2???-azobis(2-methylbutyronitrile) and 1,1???-azobis(1-cyclohexanecarbonitrile).